A technique that improves the efficiency of alveolar ventilation should decrease the pressure required and reduce the potential for lung injury during mechanical ventilation. Alveolar ventilation may be improved by replacing a portion of the anatomic dead space with fresh gas via an intratracheal catheter. We studied the effect of intratracheal gas insufflation as an adjunct to volume cycled ventilation in eight sedated, paralyzed patients with a variety of lung disorders. Continuous flows of 2, 4, and 6 L/min were delivered through a catheter positioned 1 or 10 cm above the carina. Carbon dioxide production, inspiratory minute ventilation, and peak and mean airway pressures did not change over the range of flows tested. PaCO2 and dead space volume/tidal volume decreased significantly as joint functions of catheter flow and position (p < 0.001). The highest catheter flow (6 L/min) and most distal catheter position (1 cm above the carina) were the most effective combination tested, averaging a 15% reduction in PaCO2 (range 9 to 23%). Certain characteristics of the expiratory capnogram were helpful in predicting the observed reduction in PaCO2. Tracheal gas insufflation may eventually prove a useful adjunct to a pressure-targeted strategy of ventilatory management (in either volume-cycled or pressure controlled modes), particularly when the total dead space is heavily influenced by its anatomic component.

Tracheal gas insufflation (TGI) improves the efficiency of CO2 elimination by replacing CO2 in the anatomic dead space proximal to the catheter tip with fresh gas during expiration. Turbulence generated by gas exiting the catheter tip may also contribute to alveolar ventilation. To separate distal (turbulence-related) and proximal (washout of dead space) effects of TGI, we compared the efficacy of a straight and an inverted catheter during continuous and expiratory TGI in six mechanically ventilated dogs. We reasoned that the inverted catheter cannot improve CO2 elimination from more distal conducting airways. During continuous TGI with the straight catheter, arterial PCO2 (PaCO2) decreased significantly from baseline (without TGI) of 56 +/- 10 Torr to 38 +/- 8, 36 +/- 8, and 35 +/- 8 Torr at catheter flow rates (Vcath) of 5, 10, and 15 l/min, respectively. For the same conditions, PaCO2 was always higher (P < 0.001) with the inverted catheter (42 +/- 10, 41 +/- 10, and 41 +/- 10 Torr). PaCO2 was lower with the straight (40 +/- 9 Torr) than with the inverted catheter (44 +/- 10 Torr, P < 0.001) during TGI delivered only during expiration at a Vcath of 10 l/min. End-expiratory lung volume relative to baseline increased during continuous, but not during expiratory, TGI and was significantly greater with the straight than with the inverted catheter (P < 0.0001). Our data confirm that the primary mechanism of TGI is expiratory washout of the proximal anatomic dead space but also suggest a minor contribution of turbulence beyond the tip of the straight catheter.

Increased awareness of pressure-related injury to the alveolar-capillary interface has renewed interest in modes of ventilation that limit alveolar distention such as pressure-controlled ventilation (PCV). We examined respiratory system mechanics and gas exchange during PCV in six dogs. Our data conformed to the predictions of our single-compartment mathematical model of respiratory dynamics during PCV (J Appl Physiol 1989; 67:1081-92). For a fixed pressure (Pset) and inspiratory time fraction (Tl/Ttot) (15 cm H2O and 0.3, respectively), minute ventilation (VE) reached a well-defined plateau as frequency (f) increased from 10 to 50 breaths/min and tidal volume (VT) fell progressively. Concomitantly, the physiologic dead-space fraction (VD/VT) increased from 0.50 +/- 0.04 to 0.85 +/- 0.04, and arterial PCO2 (PaCO2) rose from 39 +/- 4 to 76 +/- 12 mm Hg. At a fixed combination of frequency, applied pressure, and Tl/Ttot (40 breaths/min, 15 cm H2O, and 0.3), VE did not change when we introduced fresh gas continuously from an intratracheal catheter. However, PaCO2 and VD/VT fell progressively as catheter flow increased from zero to 14 L/min (60 +/- 12 to 40 +/- 12 mm Hg and 0.83 +/- 0.03 to 0.25 +/- 0.14 mm Hg, respectively). We conclude that during PCV at a fixed Pset and Tl/Ttot increasing frequency caused VT to fall and VE to reach a plateau. Declining VT was associated with a rise in PaCO2 because of a subsequent fall in alveolar ventilation. Insufflating fresh gas by an intratracheal catheter increased alveolar ventilation and improved CO2 elimination by washing out the anatomic dead space.(ABSTRACT TRUNCATED AT 250 WORDS)

In the setting of acute lung injury, ventilatory strategies that adjust minute ventilation (VE) to achieve eucapnia often lead to alveolar rupture or damage. Tracheal gas insufflation (TGI) reduces the VE requirements of conventional mechanical ventilation by decreasing the effective dead-space fraction (VD/VT) of each breath. We studied the effect of catheter flow rate (Vcath) and position as well as catheter tip diameter and configuration on CO2 elimination during TGI-augmented pressure-controlled ventilation (PCV) in normal dogs. We studied three catheter positions (1, 5, and 10 cm above the carina) at Vcath of 2, 5, and 10 L/min (n = 6). When the catheter tip was positioned 1 cm above the carina, PaCO2 decreased significantly from a baseline (PCV alone) of 67 +/- 10 mm Hg to 52 +/- 11, 43 +/- 9, and 32 +/- 7 mm Hg (p < 0.05) at Vcath of 2, 5, and 10 L/min, respectively. For the same Vcath values, positioning the catheter tip 10 cm above the carina increased PaCO2 to 54 +/- 15, 46 +/- 12, and 40 +/- 11 mm Hg. Advancing the catheter tip 2 cm below the carina did not improve PaCO2 significantly (n = 3). At a catheter position of 1 cm above the carina and a Vcath of 10 L/min, changing the luminal inner diameter (1.5 versus 3.0 mm) or tip configuration (open tip versus occluded tip with two side holes) of the catheter did not change PaCO2.(ABSTRACT TRUNCATED AT 250 WORDS)

Tracheal gas insufflation (TGI) improves the efficiency of CO2 elimination accomplished by conventional mechanical ventilation, primarily by reducing the anatomic (series) dead space volume. Dead space proximal to the catheter tip can be reduced by two methods. Fresh gas introduced at the carinal level during inspiration may effectively "bypass" the upper airway. Alternatively, proximal dead space can be "washed out" with fresh gas during expiration to reduce CO2 rebreathing. We examined these two modes of TGI-aided dead space reduction in nine paralyzed normal dogs receiving conventional mechanical ventilation and compared these results to those obtained with a catheter that delivered fresh gas continuously at the same flow rate, thereby accomplishing both bypass and washout. Total inspired tidal volume and cycling frequency were held constant. Differences in CO2 elimination efficiency among the TGI modes were flow dependent. Continuous catheter flow at 5 or 10 L/min reduced PaCO2 and physiologic dead space fraction (VD/VT) more than either proximal bypass or end-expiratory washout (p < 0.001). At the same catheter flow settings expiratory washout tended to improve VD/VT more than did inspiratory bypass. Under the conditions tested, constant tracheal insufflation of fresh gas improves alveolar ventilation by mechanisms that include, but are not limited to, a functional reduction in the dead space proximal to the catheter tip.